CN111239876A - Omnidirectional high chroma red structural colorants with a combination of metallic and dielectric absorber layers - Google Patents

Abstract

The present invention relates to an omnidirectional high chromatic red structural colorant having a combination of a metallic absorber layer and a dielectric absorber layer. An omnidirectional high chromatic red structural color pigment. Omnidirectional structural color pigments are in the form of a multilayer stack having a reflective core layer, a metallic absorber layer extending across the reflective core layer, and a dielectric absorber extending across the metallic absorber layer Floor. The multilayer stack reflects single-band visible light having a hue between 0-40°, and preferably between 10-30° on the a*b*Lab color map. Furthermore, the single-band visible light has a hue shift within less than 30° on the a*b*Lab colormap when viewed from all angles between 0-45° perpendicular to the outer surface of the multilayer stack shift.

Description

Omnidirectional high chromatic red with a combination of metallic and dielectric absorber layers Structural Pigment

This application is an invention patent application whose application date is June 7, 2016, the application number is 201610395759.3, and the invention name is "omnidirectional high chroma red structural colorant with a combination of a metal absorber layer and a dielectric absorber layer" divisional application.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part (CIP) of US Patent Application Serial No. 14/607,933 filed on January 28, 2015, which is in turn a Serial No. filed on August 28, 2014 CIP of US Patent Application Serial No. 14/471,834, US Patent Application Serial No. 14/471,834 and CIP of US Patent Application Serial No. 14/460,511, filed August 15, 2014, Serial No. 14/460,511 The U.S. patent application is again the CIP of the U.S. patent application serial number 14/242,429 filed on April 1, 2014, and the U.S. patent application serial number 14/242,429 is the CIP of the U.S. patent application serial number 14/242,429 filed on December 23, 2013. CIP of US Patent Application Serial No. 14/138,499, US Patent Application Serial No. 14/138,499 and CIP of US Patent Application Serial No. 13/913,402, filed June 8, 2013, Serial No. 13/913,402 The US patent application is again CIP 13/760,699 filed on February 6, 2013, and the US patent application serial number 13/760,699 is the US patent application serial number 13/572,071 filed on August 10, 2012. CIP, the entire contents of all of the above applications are incorporated herein by reference.

Field of Invention

The present invention relates to multilayer stack structures exhibiting high chromatic red color with minimal or insignificant color shift when exposed to broadband electromagnetic radiation and viewed from different angles.

Background technique

Pigments made from multilayer structures are known. In addition, pigments that exhibit or provide highly chromatic omnidirectional structural colors are also known. However, such prior art pigments require up to 39 thin film layers in order to obtain the desired color properties.

It will be appreciated that the cost associated with the preparation of thin film multilayer pigments is proportional to the number of layers required. As such, the costs associated with using multilayer dielectric material stacks to produce high chroma omnidirectional structural colors can be prohibitively high. Therefore, high chromaticity omnidirectional structural colors that require a minimum number of thin film layers would be desirable.

In addition to the above, it should also be understood that the design of pigments with red colors faces additional difficulties relative to pigments of other colors (eg, blue, green, etc.). In particular, the control of the angle-independence of the red color is difficult because of the need for thicker dielectric layers, which in turn leads to high-harmonic designs, i.e. the presence of second and possibly third harmonics is unavoidable . Also, the dark red color tone space is very narrow. As such, the red color multilayer stack has a higher angular variance.

For the above reasons, high chroma red omnidirectional structural color pigments with a minimum number of layers would be desirable.

SUMMARY OF THE INVENTION

An omnidirectional high chroma red structural color pigment is provided. The omnidirectional structural color pigment is in the form of a multilayer stack having a reflective core layer, a metallic absorber layer extending across the reflective core layer, and a dielectric absorber extending across the metallic absorber layer body layer. The multilayer stack layer reflects single-band visible light having a hue between 0-40°, and preferably between 10-30° on the a*b*Lab color map. Furthermore, the single-band visible light has a hue shift of less than 30° on the a*b*Lab colormap when viewed from all angles between 0-45° perpendicular to the outer surface of the multilayer stack, And thus provide a color shift that is not noticeable to the human eye.

The reflective core layer has a thickness between 50-200 nanometers (nm), inclusive, and can be made of reflective metals such as aluminum (Al), silver (Ag), platinum (Pt), tin (Sn), and others. combination, etc.). The reflective core layer can also be made of colorful metals such as gold (Au), copper (Cu), brass, bronze, etc.).

The metal absorber layer may have a thickness between 5-500 nm inclusive, and may be made of materials such as colored metals such as copper (Cu), gold (Au), bronze (Cu- Zn alloy), brass (Cu-Sn alloy), amorphous silicon (Si) or colored nitride materials (such as titanium nitride (TiN)). The dielectric absorber layer may have a thickness between 5-500 nm inclusive, and may be made of a dielectric producing material such as, but not limited to, iron oxide (Fe ₂ O ₃ ).

The reflective core layer, metal absorber layer and/or dielectric absorber layer may be dry deposited layers. However, the dielectric absorber layer may be a wet deposited layer. Additionally, the reflective core layer may be a central reflective core layer and the metallic absorber layer is a pair of metallic absorber layers extending across opposite sides of the central reflective core layer, ie the central reflective core layer is sandwiched between the pair of metallic absorber layers between body layers. Further, the dielectric absorber layer may be a pair of dielectric absorber layers such that the central reflective core layer and the pair of metal absorber layers are sandwiched between the pair of dielectric absorber layers.

A method of making such an omnidirectional high chroma red structural color includes: by dry depositing a reflective core layer, dry depositing a metal absorber layer extending across the reflective core layer, and dry or wet deposition across the reflective core layer The metal absorber layer extends the dielectric absorber layer to make a multilayer stack. In this manner, hybrid manufacturing methods can be used to produce omnidirectional high chroma red structural colors that can be used in pigments, coatings, and the like.

Brief Description of Drawings

Figure 1 is a schematic illustration of an omnidirectional structural color multilayer stack made of a dielectric layer, a selective absorber layer (SAL), and a reflector layer;

2A is a schematic illustration of a zero or near-zero electric field point within a ZnS dielectric layer exposed to electromagnetic radiation (EMR) having a wavelength of 500 nm;

2B is a graphical representation of the square of the absolute value of the electric field (|E| ² ) versus thickness for the ZnS dielectric layer shown in FIG. 2A when exposed to EMR at wavelengths of 300, 400, 500, 600, and 700 nm;

3 is a schematic illustration of a dielectric layer extending over a substrate or reflector layer and exposed to electromagnetic radiation at an angle θ relative to the normal direction of the outer surface of the dielectric layer;

4 is a schematic illustration of a ZnS dielectric layer with a Cr absorber layer at a zero electric field point or a near-zero electric field point within the ZnS dielectric layer for incident EMR at a wavelength of 434 nm;

5 is a graph of percent reflectance versus reflected EMR wavelength for a multilayer stack without a Cr absorber layer (eg, FIG. 2A ) and a multilayer stack with a Cr absorber layer (eg, FIG. 4 ) exposed to white light Show;

6A is a graphical representation of first and second harmonics exhibited by a ZnS dielectric layer (eg, FIG. 2A ) extending over an Al reflector layer;

Figure 6B is a multi-layer stack with a ZnS dielectric layer extending across the Al reflector layer plus a Cr absorber layer within the ZnS dielectric layer (thus absorbing the second harmonic shown in Figure 6A). Plot of percent reflectance versus reflected EMR wavelength;

Figure 6C is an illustration of a multilayer stack with a ZnS dielectric layer extending across the Al reflector layer plus a Cr absorber layer within the ZnS dielectric layer (thus absorbing the first harmonic shown in Figure 6A). Plot of percent reflectance versus reflected EMR wavelength;

7A is a graph of the squared electric field value versus dielectric layer thickness showing the electric field angle dependence of the Cr absorber layer when exposed to incident light at 0 and 45 degrees;

7B is a graphical representation of the percent absorptivity of a Cr absorber layer versus reflected EMR wavelength when exposed to white light at angles of 0° and 45° relative to the normal to the outer surface (0° being normal to the surface);

8A is a schematic illustration of a red omnidirectional structural color multilayer stack according to an aspect disclosed herein;

8B is a graphical representation of the percent absorptivity of the Cu absorber layer shown in FIG. 8A versus reflected EMR wavelength when white light is exposed to the multilayer stack shown in FIG. 8A at incident angles of 0° and 45° ;

Figure 9 is a graph comparing calculated/simulated and experimental data of percent reflectance versus reflected EMR wavelength for a proof-of-concept red omnidirectional structural color multilayer stack exposed to white light at an incident angle of 0°;

10 is a graph of percent reflectance versus wavelength for an omnidirectional structural color multilayer stack in accordance with an aspect disclosed herein;

11 is a graph of percent reflectance versus wavelength for an omnidirectional structural color multilayer stack in accordance with an aspect disclosed herein;

12 is a graphical representation of a portion of an a*b* color map using the CIELAB (Lab) color space comparing the chromaticity and hue shift of conventional paints to paints prepared from pigments according to an aspect disclosed herein (sample (b));

13 is a schematic illustration of a red omnidirectional structural color multilayer stack according to another aspect disclosed herein;

FIG. 14 is a graph of percent reflectance versus wavelength for the aspect shown in FIG. 13 .

FIG. 15 is a graph of percent absorptivity versus wavelength for the aspect shown in FIG. 13 .

FIG. 16 is a graph of percent reflectance versus wavelength versus viewing angle for the aspect shown in FIG. 13 .

FIG. 17 is a graphical representation of chromaticity and hue versus viewing angle for the aspect shown in FIG. 13 .

Figure 18 is a graphical representation of the colors reflected by the facets shown in Figure 13 versus a*b*Lab colormap; and

19 is a schematic illustration of a method for fabricating an omnidirectional red structural color multilayer stack in accordance with an aspect disclosed herein.

Detailed ways

An omnidirectional high chroma red structural color pigment is provided. The omnidirectional high chromatic red structural color is in the form of a multilayer stack having a reflective core layer, a metallic absorber layer, and a dielectric absorber layer. The metallic absorber layer extends across the reflective core layer and, in some cases, directly against or on top of the reflective core layer. The dielectric absorber layer extends across the metallic absorber layer and, in some cases, directly against or on top of the metallic absorber layer. The multilayer stack may be a symmetric stack, ie the reflective core layer is a central reflective core layer bounded by a pair of metallic absorber layers bounded by a pair of dielectric absorber layers.

The multilayer stack reflects a single band of visible light with red color having a hue between 0-40°, preferably between 10-30° on the a*b*Lab color map. Furthermore, the hue shift of the single-band visible light is less than 30° on the a*b*Lab color map when viewing the multilayer stack from all angles between 0-45° normal to its outer surface , preferably less than 20°, and more preferably less than 10°. As such, the hue shift of reflected single-band visible light may be within the 0-40° region and/or the 10-30° region on the a*b*Lab map.

The reflective core layer may be a dry deposited layer having a thickness between 50-200 nm inclusive. The term "dry deposited" means dry deposition processes such as physical vapor deposition (PVD) including electron beam deposition, sputtering, chemical vapor deposition (CVD), plasma assisted CVD, and the like. In some cases, the reflective core layer is made of reflective metals (eg, Al, Ag, Pt, Sn, combinations thereof, etc.). In other cases, the reflective core layer is made of colored metals (eg, Au, Cu, brass, bronze, combinations thereof, etc.). It should be understood that the terms "brass" and "bronze" refer to copper-zinc and copper-tin alloys, respectively, known to those skilled in the art.

The metal absorber layer may also be a dry deposited layer deposited onto the reflective core layer. In the alternative, the reflective core layer may be deposited on the metallic absorber layer. The metal absorber layer can have a thickness between 5-500 nm inclusive, and can be made of colored metals such as Cu, bronze, brass or materials such as amorphous silicon (Si), germanium (Ge), TiN, etc.). It should be understood that, for the purposes of this disclosure, the term "metal absorber layer" includes materials that are not generally considered to be metals, such as amorphous Si, Ge, TiN, and the like.

The dielectric absorber layer may also be a dry-deposited layer or a wet-deposited layer deposited onto the metal absorber layer. In the alternative, a metal absorber layer can be deposited onto the dielectric absorber layer. The dielectric absorber layer may have a thickness between 5-500 nm inclusive, and may be made of a dielectric material such as iron oxide (Fe ₂ O ₃ ), etc.). Additionally, the term "wet deposited" means a wet deposition process, such as a dissolve gel process, a spin coating process, a wet chemical deposition process, and the like.

The overall thickness of the multilayer stack may be less than 3 microns, preferably less than 2 microns, more preferably less than 1.5 microns, and still more preferably less than or equal to 1.0 microns. Furthermore, the multilayer stack has a total number of layers less than or equal to 9, preferably a total number of layers less than or equal to 7, and more preferably a total number of layers less than or equal to 5.

Referring to Figure 1, a design is shown in which the underlying reflector layer (RL) has a _first dielectric material layer DL1 extending across the reflector layer and _a selective absorber layer extending across the DL1 layer sal. Additionally, another DL ₁ layer may or may not be provided and may or may not extend across the selective absorber layer. Also shown in this figure is an illustration of the reflection or selective absorption of all incident electromagnetic radiation by the multilayer structure.

As illustrated in Figure 1, such designs correspond to different approaches for designing and manufacturing the desired multilayer stack. In particular, the thickness of a zero-energy point or a point near zero-energy for a dielectric layer is used and discussed below.

For example, Figure 2A is a schematic illustration of a ZnS dielectric layer extending across an Al reflector core layer. The ZnS dielectric layer has a total thickness of 143 nm, and for incident electromagnetic radiation having a wavelength of 500 nm, a zero or near-zero energy point exists at 77 nm. In other words, for incident electromagnetic radiation (EMR) with a wavelength of 500 nm, the ZnS dielectric layer exhibits a zero or near-zero electric field at a distance of 77 nm from the Al reflector layer. Furthermore, Figure 2B provides a graphical representation of the energy field across the ZnS dielectric layer for several different incident EMR wavelengths. As shown in the figure, the dielectric layer has a zero electric field at 77 nm thickness for 500 nm wavelength, but a non-zero electric field at 77 nm thickness for EMR wavelengths of 300, 400, 600 and 700 nm.

Regarding the calculation of the zero electric field point or near zero electric field point, Figure 3 illustrates a dielectric layer 4 with a total thickness "D", an incremental thickness "d", and a refractive index "n", which is located in a position with a refractive index _ns on the substrate or core layer 2 . Incident light strikes the outer surface 5 of the dielectric layer 4 at an angle θ with respect to a line 6 perpendicular to the outer surface 5 and is reflected from the outer surface 5 at the same angle θ. Incident light is transmitted through the outer surface 5 and into the dielectric layer 4 at an angle θ _F relative to the line 6 and impinges on the surface 3 of the substrate layer 2 at an angle θ _s .

For a single dielectric layer, θ _s = θ _F and the energy/electric field (E) can be expressed as E(z) when z=d. According to Maxwell's equation, for s-polarization, the electric field can be expressed as:

And for p-polarization, it can be expressed as:

并且λ为将要反射的所期望的波长。此外，α＝n_s sin θ_s，其中“s”对应于图5中的基材，并且

为作为z的函数的所述层的介电常数。如此，对于s极化in

And λ is the desired wavelength to be reflected. Furthermore, α= _ns sin θ _s , where “s” corresponds to the substrate in FIG. 5 , and

is the dielectric constant of the layer as a function of z. So, for s-polarization

|E(d)| ² = |u(z)| ² exp(2ikαy)| _z = _d (3)

and for p polarization

It should be understood that the variation of the electric field along the Z direction of the dielectric layer 4 can be estimated by calculating the unknown parameters u(z) and v(z), which can be shown as:

Naturally, "i" is the square root of -1. Using the boundary conditions u| _z=0 =1, v| _z=0 = _qs , and the following relation:

For s-polarization, q _s = _ns cos θ _s (6)

For p-polarization, q _s = _ns /cos θ _s (7)

For s-polarization, q=n cos θ _F (8)

For p-polarization, q=n/cos θ _F (9)

u(z) and v(z) can be expressed as:

and

的s极化：Therefore, for

The s-polarization of:

and for p-polarization:

in:

α=n _s sin θ _s =n sin θ _F (15)

and

并且α＝0：Thus, for the simple case of θ _F = 0 or normal incidence,

and α=0:

其允许解出厚度“d”，即介电层内电场为零的位置或地点。s-polarized |E(d)| ² =p-polarized

It allows to solve for thickness "d", ie the position or location within the dielectric layer where the electric field is zero.

Referring now to Figure 4, Equation 19 was used to calculate the zero or near-zero electric field point in the ZnS dielectric layer shown in Figure 2A when exposed to EMR at a wavelength of 434 nm. This zero or near-zero electric field point is calculated to be 70 nm (for 500 nm wavelength, substitute 77 nm). Furthermore, a 15 nm thick Cr absorber layer was inserted at a thickness or distance of 70 nm from the Al reflector core layer to provide a zero or near-zero electric field ZnS-Cr interface. Such an inventive structure allows light with a wavelength of 434 nm to pass through the Cr-ZnS interface, but absorbs light that does not have a wavelength of 434 nm. In other words, the Cr-ZnS interface has zero or near-zero electric field for light with a wavelength of 434 nm, and thus 434 nm light passes through this interface. However, the Cr-ZnS interface does not have a zero or near-zero electric field for light with wavelengths other than 434 nm, and thus, such light is absorbed by the Cr absorber layer and/or the Cr-ZnS interface, and not by the Al reflector layer reflection.

It should be understood that some percentage of the light in the +/- 10 nm range of the desired 434 nm will pass through the Cr-ZnS interface. However, it should also be understood that such a narrow band of reflected light, eg, 434 +/- 10 nm, would still provide a dazzling structural color to the human eye.

The results for the Cr absorber layer in the multilayer stack of FIG. 4 are illustrated in FIG. 5 , where percent reflectance versus reflected EMR wavelength is shown. As shown by the dashed line, which corresponds to the ZnS dielectric layer shown in Figure 4 without the Cr absorber layer, a narrow reflection peak exists at about 400 nm, but a much broader peak exists at about 550+ nm. In addition, in the wavelength region of 500 nm, there is still a large amount of reflected light. As such, there are double peaks that prevent the multilayer stack from having or exhibiting structural color.

In contrast, the solid line in FIG. 5 corresponds to the structure shown in FIG. 4 where the Cr absorber layer is present. As shown in the figure, there is a sharp peak at about 434 nm and a sharp drop in reflectivity for wavelengths greater than 434 nm is provided by the Cr absorber layer. It should be understood that the sharp peaks represented by the solid lines appear visually as dazzling/structural colors. Furthermore, Figure 5 depicts the measurement of the width of the reflection peak or frequency band, ie the width of the frequency band (also known as the width at half maximum (FWHM)) is determined at 50% reflectivity of the wavelength of maximum reflection.

Regarding the omnidirectional behavior of the multilayer structure shown in Figure 4, the thickness of the ZnS dielectric layer can be designed or set such that only the first harmonic of the reflected light is provided. It should be understood that this is sufficient for a "blue" color, however, the production of a "red" color requires other conditions. For example, the angle-independent control of the red color is difficult because of the need for thicker dielectric layers, which in turn leads to high-harmonic designs, ie the presence of second and possibly third harmonics is unavoidable. Also, the dark red color tone space is very narrow. As such, the red-colored multilayer stack has higher angular dispersion.

To overcome the higher angular dispersion of red color, the present application discloses a unique and novel design/structure that provides angle independent red color. For example, Figure 6A illustrates a dielectric layer exhibiting first and second harmonics to incident white light when the outer surface of the dielectric layer is viewed from 0 and 45° relative to the normal to the outer surface. As shown by the illustration, the low angular dependence (small Δλ _c ) is provided by the thickness of the dielectric layer, however, such a multilayer stack has blue color (first harmonic) and red color ( 2nd harmonic), and therefore does not apply to the desired "red only" color. Therefore, concepts/structures have been developed that use absorber layers to absorb unwanted harmonic series. FIG. 6A also illustrates an example of the location of the center wavelength (λ _c ) of the reflection band for a given reflection peak, and the dispersion or shift (Δλ _c ) of the center wavelength when viewing the sample from 0 and 45°.

Turning now to Figure 6B, the second harmonic shown in Figure 6A is absorbed with a Cr absorber layer at the appropriate dielectric layer thickness (eg, 72 nm) and provides a dazzling blue color. Furthermore, Figure 6C depicts the provision of red color by absorbing the first harmonic with a Cr absorber at different dielectric layer thicknesses (eg, 125 nm). However, Figure 6C also illustrates that the use of a Cr absorber layer can result in an angular dependence greater than that desired for the multilayer stack, ie, greater than the desired _Δλc .

It should be understood that the relatively large shift in _λc for red colors compared to blue colors is due to the very narrow hue space of dark red colors and the fact that Cr absorber layer absorption is related to a non-zero electric field wavelength, i.e. light that does not absorb when the electric field is zero or close to zero. As such, Figure 7A depicts that the zero or non-zero point is different for light wavelengths at different angles of incidence. Such factors lead to the angle-dependent absorption shown in Figure 7B, ie, the difference in the 0° and 45° absorptivity curves. Therefore, to further refine the multilayer stack design and angle-independent performance, absorber layers are used that absorb, for example, blue light, regardless of whether the electric field is zero or not.

In particular, Figure 8A shows a multilayer stack with a Cu absorber layer extending across the dielectric ZnS layer in place of the Cr absorber layer. The results of using such a "colored" or "selective" absorber layer are shown in Figure 8B, which demonstrate the" tighter" clusters. As such, the comparison between Figures 8B and 7B illustrates the dramatic improvement in the angular independence of absorptivity when a selective absorber layer is used instead of a non-selective absorber layer.

Based on the above, a proof-of-concept multilayer stack structure was designed and fabricated. In addition, the calculated/simulated results of the samples used for the proof-of-concept were compared with the actual experimental data. In particular, and as shown by the graph in Figure 9, a dazzling red color was produced (wavelengths greater than 700 nm are typically not seen by the human eye), and in calculations/simulations and experimental light obtained from actual samples Very good consistency is obtained between the data. In other words, calculations/simulations may be used and/or used to simulate multilayer stack designs according to one or more embodiments disclosed herein and/or results of prior art multilayer stacks.

Figure 10 shows a plot of percent reflectance versus reflected EMR wavelength for another omnidirectional reflector design when exposed to white light at angles of 0° and 45° relative to the normal of the reflector's outer surface. As shown in this graph, both the 0° and 45° curves illustrate the very low reflectivity (eg, less than 10%) provided by the omnidirectional reflector for wavelengths less than 550 nm. However, as shown in this curve, the reflector provides a dramatic increase in reflectivity at wavelengths between 560-570 nm and reaches a maximum of about 90% at 700 nm. It should be understood that the portion or region of the graph on the right hand side (IR side) of the curve represents the IR portion of the reflection band provided by the reflector.

The dramatic increase in reflectivity provided by the omnidirectional reflector is characterized by the UV side edge of each curve extending from a low reflectivity portion with wavelengths less than 550 nm to a high reflectivity portion (eg greater than 70%). The linear portion 200 of the UV side edge is inclined at an angle (β) greater than 60° with respect to the x-axis, has a length L of about 40 on the reflectivity axis, and a slope of 1.4. In some cases, the linear portion is inclined at an angle greater than 70° with respect to the x-axis, while in other cases, β is greater than 75°. Additionally, the reflection band has a visible FWHM of less than 200 nm, and in some cases a visible FWHM of less than 150 nm, and in other cases a visible FWHM of less than 100 nm. Furthermore, the center wavelength λ _c of the visible reflection band as illustrated in FIG. 10 is defined as the wavelength equidistant between the UV side edge of the reflection band and the IR edge of the IR spectrum at the visible FWHM.

It should be understood that the term "visible FWHM" means the width of the reflection band between the UV side edge of the curve and the edge of the IR spectral range, beyond which the reflection provided by the omnidirectional reflector is not visible to the human eye. In this manner, the inventive designs and multilayer stacks disclosed herein use the invisible IR portion of the electromagnetic radiation spectrum to provide dazzling or structural colors. In other words, despite the fact that the reflector can reflect a wider frequency band of electromagnetic radiation extending into the IR region, the omnidirectional reflector disclosed herein utilizes the invisible IR portion of the electromagnetic radiation spectrum to provide a narrow band of electromagnetic radiation reflected visible light.

Referring now to FIG. 11 , a plot of percent reflectance versus wavelength for another seven-layer design omnidirectional reflector is shown when exposed to white light at angles of 0° and 45° relative to the reflector surface. Furthermore, a definition or characterization of the omnidirectional properties provided by the omnidirectional reflectors disclosed herein is shown. In particular, and when the reflection band provided by the reflectors of the present invention has a maximum value or peak as shown, each curve has a center wavelength (λ _c ), which is defined as the wavelength that exhibits or experiences maximum reflectivity. The term wavelength of maximum reflection can also be used for λ _c .

As shown in FIG. 11 , when the outer surface of the omnidirectional reflector is observed from an angle of 45° (λ _c (45°)), for example, the outer surface is inclined by 45° with respect to the human eye observing the surface, compared with the angle from 0° ( There is a shift or displacement of λ _c compared to λ _c (0°)), ie when the surface is viewed perpendicular to the surface. This shift in λ _c (Δλ _c ) provides a measure of the omnidirectional nature of the omnidirectional reflector. Naturally, zero offset, ie no offset at all, would be a perfect omnidirectional reflector. However, the omnidirectional reflector disclosed herein can provide a Δλ _c of less than 50 nm, which can appear to the human eye as if the surface of the reflector has not changed color, and thus from a practical point of view the reflector is omnidirectional of. In some cases, the omnidirectional reflectors disclosed herein can provide a Δλ _c of less than 40 nm, in other cases a Δλ _c of less than 30 nm, and in still other cases a Δλ _c of less than 20 nm, while still Other cases may provide Δλ _c of less than 15 nm. Such a shift in _Δλc can be determined from a plot of the reflector's actual reflectivity versus wavelength, and/or alternatively, by modeling the reflector if the material and layer thickness are known.

Another definition or characterization of the omnidirectional nature of a reflector can be determined by the offset of the side edges of a given set of angular reflection bands. For example, and referring to Figure 11, the reflectivity for an _{omnidirectional} reflector viewed from 0° ( The shift or displacement (ΔS _UV ) of the _UV side edge in terms of SUV (0°)) provides a measure of the omnidirectional nature of the omnidirectional reflector. It will be appreciated that the shift of the UV side edge (ΔS _UV ) is measured at the visible FWHM and/or the shift of the UV side edge (ΔS _UV ) can be measured at the visible FWHM.

Naturally, zero offset, ie no offset at all (ΔS _UV = 0 nm) would characterize a perfect omnidirectional reflector. However, the omnidirectional reflector disclosed herein can provide a ΔS _UV of less than 50 nm, which can appear to the human eye as if the surface of the reflector has not changed color, and thus, from a practical standpoint, the reflector is omnidirectional of. In some cases, the omnidirectional reflectors disclosed herein can provide an ΔS _UV of less than 40 nm, in other cases an ΔS _UV of less than 30 nm, and in still other cases an ΔS _UV of less than 20 nm, while still Others may provide a ΔS _UV of less than 15 nm. Such a shift in ΔS _UV can be determined from a plot of the reflector's actual reflectivity versus wavelength, and/or alternatively, by modeling the reflector if the material and layer thickness are known.

The shift in omnidirectional reflection can also be measured by a low hue shift. For example, as shown in FIG. 12 (see, eg, Δθ ₁ ), pigments prepared from multilayer stacks according to aspects disclosed herein have a hue shift of 30° or less, and in some cases, hue The offset is 25° or less, preferably less than 20°, more preferably less than 15°, and still more preferably less than 10°. In contrast, conventional pigments exhibit a hue shift of 45° or more (see, eg, Δθ ₂ ). It will be appreciated that the hue shift associated with Δθ ₁ generally corresponds to a red color, however a low hue shift is relevant for any color reflected by the hybrid omnidirectional structural color pigments disclosed herein.

A schematic illustration of an omnidirectional multilayer stack according to another aspect disclosed herein is shown at 10 in FIG. 13 . The multilayer stack 10 has a first layer 110 and a second layer 120 . An optional reflector layer 100 may be included. Exemplary materials for the reflector layer 100 (sometimes referred to as the reflector core layer) may include, but are not limited to, Al, Ag, Pt, Cr, Cu, Zn, Au, Sn, and combinations or alloys thereof. As such, the reflector layer 100 may be a metallic reflector layer, although this is not required. Additionally, exemplary thicknesses of the core reflector layer are between 30 and 200 nm.

A symmetrical pair of layers may be located on opposite sides of the reflector layer 100, ie the reflector layer 100 may have another first layer disposed opposite the first layer 110, sandwiching the reflector layer 100 between the pair of first layers between. Furthermore, another second layer 120 may be arranged opposite to the reflector layer 100, thereby providing a five-layer structure. Therefore, it should be understood that the discussion of multilayer stacks provided herein also includes the possibility of mirrored structures with respect to one or more central layers. As such, Figure 13 may be an illustration of one half of a five-layer multilayer stack.

With respect to the aspects discussed above, the first layer 110 may be an absorber layer, eg, a metal absorber layer having a thickness between 5-500 nm inclusive. Also, the second layer may be a dielectric absorber layer having a thickness between 5-500 nm inclusive. The metal absorber layer 110 may be made of colored metallic materials such as Cu, bronze, brass, or materials such as amorphous Si, Ge, TiN, and combinations thereof. The dielectric absorber layer 120 may be made of Fe ₂ O ₃ .

Aspects as shown in FIG. 13 and having dimensions as shown in Table 1 below exhibit the reflectance spectrum shown in FIG. 14 . As shown in this figure, Cu or its alloys or other colored reflectors such as TiN layer 110 and _Fe2O3 dielectric absorber layer 120 having the thicknesses shown in Table ₁ provide a reflectance spectrum where less than about 550 A wavelength of -575 nm has a reflectivity of less than 10-15%, and wavelengths greater than approximately 575-600 nm correspond to between 0-40°, preferably between 10-30° on the a*b*Lab color map between hues. Further, the chromaticity of the reflection band of visible light is greater than 70, preferably greater than 80, and more preferably equal to or greater than 90.

Table 1

<u>Layer</u> <u>Materials</u> <u>Thickness(nm)</u> 100 Al 80.0 110 Cu or alloys such as brass, bronze, etc. 184.5 120 Fe₂O₃ 28.6

The reflectance spectrum of such a multilayer stack as shown in FIG. 13 is exemplarily shown in FIG. 14 for viewing angles of 0° and 45°. As shown in this figure, the offset of the UV side edge at FWHM (ΔS _UV = _SUV (0°) − _SUV (45°)) is less than 50 nm, preferably less than 30 nm, and still more preferably less than 20 nm, and also More preferably less than 10 nm. Combined with the width of the frequency bands in the visible spectrum, a shift in the reflected frequency band between angles 0 and 45° corresponds to a color change that is insignificant to the human eye.

FIG. 15 shows absorption versus wavelength for the design shown in FIG. 13 . As shown in this figure, the multilayer stack 10 absorbs over 80% of the visible light spectrum for wavelengths up to about 575 nm. Furthermore, this aspect 10 absorbs greater than 40% of all wavelengths up to about 660 nm. As such, the combination of the metallic absorber layer 110 and the dielectric absorber layer 120 provides a visible reflection band having between 0-40°, and preferably between 10-30° in the a*b*Lab color space Hue, that is, the wavelength of reflection in the red color spectrum.

Figure 16 shows a graph of this aspect 10 as a function of percent reflectance, wavelength reflected, and viewing angle. As shown in this 3D contour plot, the reflectance is very low, i.e. less than 20% for wavelengths between 400-550-575nm and at viewing angles between 0 and 45-50° . However, there is a sharp increase in percent reflectance at wavelengths between about 550-600 nm.

a*和b*是当暴露于宽频带电磁辐射(例如白光)时，由多层堆叠体反射的颜色在Lab色空间或映射上的坐标。Another method or technique to describe the omnidirectional properties of the inventive multilayer stacks disclosed herein is a graph of chromaticity and hue versus viewing angle as shown in FIG. 17 . Figure 17 shows the reflection characteristics of the aspect shown in Figure 13, where the hue for angles between 0 and 45° is between 20-30 with less than 10°, preferably less than 5° of change or offset shift. Also, for all viewing angles between 0-45°, chromaticity is between 80-90, where chromaticity (C*) is defined as

a* and b* are the coordinates on the Lab color space or map of the colors reflected by the multilayer stack when exposed to broadband electromagnetic radiation (eg, white light).

Figure 18 shows or plots the hue of the aspect shown in Figure 13 on the a*b*Lab color space map (see data points indicated by arrows). The area between 15-40° is also shown on this map. It should be understood that these two points are used to illustrate a 0° viewing angle relative to the normal to the outer surface of the multilayer stack. Additionally, it should be understood that between viewing angles of 0-45°, the hue of this aspect as shown in FIG. 13 does not move outside the 15-40° hue region. In other words, this aspect exhibits a low hue shift, eg less than 30°, preferably less than 20°, and still more preferably less than 10°. It will further be appreciated that the aspect shown in Figure 13 can also be designed to provide single-band visible light having a hue between 0-40°, and can be plotted in Figure 18, and preferably has a range between 0-40°. Single-band visible light with hues between 10-30°.

Turning now to FIG. 19 , a method for making an omnidirectional high chroma red structural color is shown generally at 20 . The method 20 includes dry depositing a reflective core layer at step 202 and then dry depositing a metal absorber layer on the dry deposited reflective core layer at step 210 . The dielectric absorber layer is then dry deposited or wet deposited onto the metal absorber layer at step 220 . It should be understood that steps 210 and 220 may be repeated to create additional layers on the dry deposited reflective core layer. In addition, a dry deposited reflective core layer can be deposited onto the metal absorber layer, and a wet deposited dielectric layer can also be deposited onto the metal absorber layer.

The above embodiments and aspects are for illustrative purposes only, and variations, alterations, etc. will be apparent to those skilled in the art and still fall within the scope of the invention. As such, the scope of the invention is defined by the claims and all equivalents thereof.

Claims (19)

1. Omnidirectional high chroma red structural pigment, including: A multilayer stack having: Reflective core layer; a metallic absorber layer extending across the reflective core layer; and a dielectric absorber layer extending across the metallic absorber layer; The multilayer stack reflects single-band visible light having a center wavelength between 500-700 nanometers and a visible FWHM of less than 200 nm. 2. The omnidirectional high-chroma red structural colorant of claim 1, wherein the multilayer stack reflects single-band visible light having a hue between 0-40° on the a*b*Lab color map, The single-band visible light has on the a*b*Lab colormap a value between 0-45° when viewed from all angles between 0-45° normal to the outer surface of the multilayer stack Hue shift within 40°. 3. The omnidirectional high chroma red structural colorant of claim 1, wherein the reflective core layer has a thickness between 50-200 nanometers, inclusive. 4. The omnidirectional high chromaticity red structural colorant of claim 3, wherein the reflective core layer is made of a reflective metal selected from the group consisting of Al, Ag, Pt, Sn, and combinations thereof. 5. The omnidirectional high chroma red structural colorant of claim 3, wherein the reflective core layer is made of a colored metal selected from the group consisting of Au, Cu, brass, bronze, and combinations thereof. 6. The omnidirectional high chroma red structural colorant of claim 3, wherein the metallic absorber layer has a thickness between 5-500 nanometers, inclusive. 7. The omnidirectional high chroma red structural colorant of claim 6, wherein the metallic absorber layer is made of Cu, bronze, brass, amorphous Si, Ge, TiN, and combinations thereof. 8. The omnidirectional high chroma red structural colorant of claim 6, wherein the dielectric absorber layer has a thickness between 5-500 nm, inclusive. 9. The omnidirectional high chroma red structural colorant of claim ₈ , wherein the dielectric absorber layer is made of _Fe2O3 . 10. The omnidirectional high chroma red structural colorant of claim 6, wherein the reflective core layer is a central reflective core layer and the metallic absorber layer is a layer extending across opposite sides of the central reflective core layer. For metal absorber layers, the central reflective core layer is sandwiched between the pair of metal absorber layers. 11. The omnidirectional high chroma red structural colorant of claim 10, wherein the dielectric absorber layer is a pair of dielectric absorber layers, the central reflective core layer and the pair of metal absorber layers sandwiched between the pair of dielectric absorber layers. 12. A method for preparing an omnidirectional high-chroma red structural colorant, the method comprising: Multilayer stacks are fabricated by: Dry deposition of reflective core layer; dry depositing a metal absorber layer extending across the reflective core layer; dry or wet depositing a dielectric absorber layer extending across the metallic absorber layer; and The multilayer stack reflects single-band visible light having a center wavelength between 500-700 nanometers and a visible FWHM of less than 200 nm. 13. The method of claim 12, wherein the multilayer stack reflects visible light having a hue between 0-40° on the a*b*Lab color map, and on the a*b*Lab color map Has a hue shift between this 0-40°. 14. The method of claim 12, wherein the reflective core layer has a thickness between 50-200 nanometers, inclusive. 15. The method of claim 14, wherein the reflective core layer is made of a reflective metal selected from the group consisting of Al, Ag, Pt, Sn, and combinations thereof. 16. The method of claim 14, wherein the reflective core layer is made of a colored metal selected from the group consisting of Au, Cu, brass, bronze, and combinations thereof. 17. The method of claim 14, wherein the metal absorber layer has a thickness between 5-500 nanometers, inclusive. 18. The method of claim 17, wherein the metal absorber layer is made of the following materials: Cu, bronze, brass, amorphous Si, Ge, TiN, and combinations thereof. 19. The method of claim 17, wherein the dielectric absorber layer has a thickness between 5-500 nanometers, inclusive, and is made _{of Fe2O3} .

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